This invention relates in general to systems for finding profiles of topographical features of small dimensions, such as those of a diffracting grating, and in particular to such systems using optical spectroscopic techniques.
As the integration density and speed of microelectronic devices increase, circuit structures continue to shrink in dimension size and to improve in terms of profile edge sharpness. The fabrication of state-of-the-art devices requires a considerable number of process steps. It is becoming increasingly important to have an accurate measurement of submicron linewidth and quantitative description of the profile of the etched structures on a pattern wafer at each process step. Furthermore, there is a growing need for wafer process monitoring and close-loop control such as focus-exposure control in photolithography.
Spectroscopic diffraction-based techniques are especially well suited for microelectronics metrology applications because they are nondestructive, sufficiently accurate, repeatable, rapid, simple and inexpensive relative to critical dimension-scanning electron microscopy. In such diffraction-based analysis techniques, typically a model of the profile is first constructed, where the model includes a number of parameters that can be varied. One or more diffraction intensity versus wavelength curves are calculated based on the model constructed and the curve(s) are compared with measured diffraction data from the sample. The parameters are then adjusted until a match is found between the curve(s) and the measured data.
The current methods being used include multi-slab models where a number of rectangular or trapezoidal slabs are put on top of one another to form a seed profile that is an approximation of the profile being measured. The parameters that can be adjusted include width and height of the rectangles or width, height and sidewall angle of the trapezoids. It is found that in the wafer processing processes, a number of very different profiles of structures may be encountered. The current methods are inadequate for measuring a wide variety of very different profiles in the manufacturing process. A simple increase of the number of slabs to model such variety of profiles requires the generation of huge libraries whose size grows exponentially with the number of slabs and the associated parameters. Furthermore, different sets of parameters, corresponding to different profiles, can produce indistinguishable spectroscopic data, resulting in a problem known as cross-correlation.
In U.S. Pat. No. 5,963,329, Conrad et al. proposed an improved method to measure actual profiles. In this model, the number of independent parameters or variables is reduced by adopting particular profile shapes such as a “S” line profile, by dividing the model line profile into two or more sub-profiles and providing a numerical model of each sub-profile so that fewer scaling factors may be used to adjust all slab widths and heights within the single sub-profile.
While the above-described method of Conrad et al. reduces the number of parameters that one needs to contend with, this method still has some drawbacks. Thus, it cannot be used for measuring line profiles made of more than material, and for measuring optical parameters as well as geometric parameters. It is therefore desirable to provide an improved model that can be used for determining the above mentioned samples in a manner so that the solution converges to a single solution without a high risk of cross-correlation.
As noted above, the shapes of line profiles encountered on semiconductor wafers during fabrication can take on a wide variety of shapes. Such line profiles are typically situated on and/or below layers of materials which may be the same as or different from the material of the profiles. When diffraction-based spectroscopic techniques are used to measure such profiles, the radiation used in the technique would interact with the one or more layers and transmitted or reflected radiation from the layers is detected by the detectors that are used for detecting radiation from the line profile. Where it is not possible or very difficult to separate the contribution of the signal due to the layers from the contribution of the signal due to the line profile, it is desirable for any technique used to measure the parameters of such layers simultaneously with measurement of the line profile. None of the existing techniques has such capability. It is therefore desirable to provide an improved system where the contribution of such layers to the detector signal can be taken into account.
Currently in the market the common methods of determining a profile (cross section) of a structure are: scanning electron microscopy or SEM (cross section and top down), atomic force microscopy or AFM, and scatterometry. For production monitoring scatterometry is being established as the leading method for lot by lot monitoring using periodic test targets.
The basic methodology in scatterometry is the comparison of the measured (typically spectral) data to a library that has been prepared in advance and contains the possible variations of the target profile AND underlying layers. However, in many situations the number of variables (e.g. underlying layer thickness in a damascene layer) is prohibitively large and therefore prevent the user from creating a library.
U.S. Pat. No. 5,963,329 describes the use of a real time regression algorithm for the determination of the grating profile using its measured spectral reflected intensity. A major difficulty with this algorithm is that the regression time becomes prohibitive for more than 4 degrees of freedom (floating parameters such as the CD, side wall angle and underlying film thickness). This prevents the user from using this methodology for the measurements of damascene structures or even photo-resist on complex/variable films. In addition, the added number of degrees of freedom results in a non-robust root convergence that will tend to lock onto local correlated minima.
The major disadvantages of the above methods are as follows. It is difficult to create a library for gratings on multi variable films (e.g. photo resist on damascene layer or etched trenches or vias in inter-metal dielectrics). It is also difficult to regress in real time on more than 4 floating profile and film variables. It is therefore desirable to provide an improved system to alleviate such problems.